Interpretive Summary: Honey bees face numerous pests and pathogens, leading to high costs in the maintenance of this important agricultural species. One route toward breeding bees resistant to these pathogens involves enhancing the immune system of bees through selection. This study looks at immune response of bees when experimentally fed the bacterium responsible for the important disease American Foulbrood. It is shown for the first time that bees do have an immune response to this bacterium and that this response is variable across different bees. One antibiotic protein produced by bees seems to be a particularly good measure of the bee immune response. The described techniques can be used by governmental and university researchers to characterize the immune responses of different bees and use this information in breeding schemes for new ways to combat disease.

Technical Abstract:
Honey bee larvae are highly susceptible to the bacterial pathogen Paenibacillus larvae only during the first instar of larval development. Transcript levels for genes encoding two antimicrobial peptides, abaecin and defensin, as well as for two candidate proteins for the immune-response cascade (PGRP-LD and masquerade) were compared for control and inoculated bees throughout larval development. Transcripts for all four are present throughout development. This suggests that other physiological or dietary factors may better explain the age-based change in vulnerability to this pathogen. One of these peptides, abaecin, shows significant up-regulation 24 hours following oral inoculation with P. larvae, precisely when this bacterium begins invading the midgut epithelia of bees. Expression of both antimicrobial peptides varied tremendously across different bees, suggesting an allelic component to their expression. The implications of this result for current hypotheses related to disease tolerance in social insects are discussed, along with implications for breeding bees resistant to this important disease.